Nonevaporable getters are porous metal structures that are used widely for vacuum maintenance and for purification of inert gasses. Getters operate by means of strong surface chemisorption of reactive gasses, for example H.sub.2, CO, CO.sub.2, H.sub.2 O.sub.2, N.sub.2, etc. In the case of H.sub.2, in addition to dissociative surface chemisorption, H atoms actually enter metal crystal lattices to form interstitial solutions and often hydride phases. For all other reactive gasses, gettering reactions are limited largely to metal surfaces, at least at near ambient temperatures. Therefore, from reactivity and capacity points of view, it is a desirable feature that getter structures be porous and have high surface areas. This feature means that getters are made almost universally by powder metallurgy processes. In addition it is important usually that getter structures be tough mechanically so as to resist breakage and particulation during rough service. Ease of making powder usually requires alloy brittleness; conversely, a getter structure requires a degree of alloy toughness. As will be seen, it is one of the objects of this invention to avoid this apparent metallurgical dilemma.
Alloys and intermetallic compounds based on the elements zirconium and vanadium have been shown to have desirable gettering properties. In 1966, Pebler and Gulbransen, in an article published in ELECTROCHEMICAL TECHNOLOGY, Vol.4, No. 5-6, pp. 211-215, showed that the intermetallic compound ZrV.sub.2 had good room temperature solubility for hydrogen. Later, in 1979 U.S. Pat. No. 4,163,666, Shaltiel, Davidov, and Jacob disclosed the H.sub.2 gettering potential of Fe substituted versions of ZrV.sub.2, i.e. intermetallic compounds of the general formula Zr(V.sub.1-x Fe.sub.x).sub.2. The above-mentioned intermetallic compounds apparently are brittle and easily grindable by mechanical means into powder, but as mentioned above a brittle material would not be expected to allow manufacture of a tough sintered structure. Also the vanadium contents of these materials are high, 26 to 53 wt.% V. In the case of H.sub.2 gettering, increasing V levels in Zr-V alloys tends to lower undesirably the saturation level at some given temperature and pressure. Also increasing V levels increase raw materials costs. Thus it would be desirable to minimize V content and maximize Zr content.
One attempt to achieve the foregoing objective was reported by Mendelsohn and Gruen in 1982 U.S. Pat. No. 4,360,445. Those inventors prepared oxygen stabilized compounds containing for example Zr, V and Fe, among others. A particular example shown was Zr.sub.1.4 V.sub.0.5 Fe.sub.0.5 O.sub.0.25. While this compound contains only about 14 wt.% V and 69 wt.% Zr, it contains a rather large amount of Fe (15 wt.%), which the inventors say "increases the brittleness to permit fracturing and powdering of the alloy". Because Fe has a low affinity for hydrogen, large amounts of Fe would be expected undesirably to decrease hydrogen gettering ability. In fact the hydrogen capacities for the aforementioned Oxygen stabilized compounds are not particularly high at low pressure.
The heretofore most successful approach to Zr-V type getters was reported by Boffito, Barosi and Figini in 1982 U.S. Pat. No. 4,312,669. These inventors patented a family of Zr-V-Fe ternary gettering alloys. The most prominent of these alloys is, by weight percent, 70 Zr-24.6 V-5.4 Fe, which has been sold successfully under the Trademark St-707 by SAES Getters S.p.A of Milan, Italy. According to a 1984 paper by Boffito, Doni and Rosai, published in the JOURNAL OF LESS-COMMON METALS, Vol. 104, pp. 149-157, th is alloy preferably con sists of the phases Zr (V.sub.0.83 Fe.sub.0.17).sub.2 and .alpha.-Zr. That is, the alloy family is considerably more rich in Zr than the Zr(V,Fe).sub.2 intermetallic which results in better H.sub.2 and H.sub.2 O gettering characteristics. In addition the presence of ductile .alpha.-Zr phase helps to increase mechanical toughness. However, U.S. Pat. No. 4,312,669 limits Zr content to no more than 75 wt.% because the alloy "could become too plastic creating difficulties in its transformation into a fine powder". Additionally, in a 1981 patent (U.S. Pat. No. 4,269,624) on the manufacture of such ternary gettering alloys, Figini disclosed conventional melting followed by mechanical grinding (ball milling) with all examples and claims such that the Zr contents are equal to or less than 75 wt.%. Thus all prior art of which we are knowledgeable, suggest that practically it is not possible by prior art methods to produce powder for the manufacture of Zr-V type gettering alloys wherein the Zr content is greater than 75 wt.%.
The aforementioned grindable St-707, 70 Zr-24.6 V-5.4 Fe alloy can be made stronger in final getter form by mixing elemental Zr powder with the pre-ground alloy powder before final sintering, as described in a brochure entitled St 172 ADVANCED POROUS GETTERS published May, 1987, by SAES Getters S.p.A. This strengthening is helpful but retains certain disadvantages. First, in our experience such mechanical mixtures of powder do not result in as strong sintered structures as when the increased Zr level is incorporated metallurgically during the original alloy melting, as is the case for the present invention. This feature may be a desired result of the more intimate mixture of phases that can occur during melt alloying versus mechanical blending. Second, elemental Zr powder is much more expensive than bulk Zr sponge which is added during original alloy melting. Third, the Zr blending procedure represents an extra production step which must be done carefully in an inert atmosphere, because of potential powder flammability.
As will be shown, the present invention eliminates completely the problem of making fine powder from high Zr gettering alloys, especially those of the Zr-V type where Zr is greater than 75 wt.%, i.e. those alloys that cannot be pulverized practically by mechanical means. We have found such alloys can be pulverized conveniently by simple and direct reaction of the metal ingot, or ingot pieces, with hydrogen gas, resulting in hydrogen absorption, alloy embrittlement, hydride formation, and crystal lattice expansion, all of which in sum result in spontaneous decrepitation of the entire ingot into powder and/or small granules. The hydrogen so introduced can be largely removed subsequently by vacuum/thermal means to result in low hydrogen alloy powder for use in the manufacture of sintered porous getters or used directly for gettering purposes.
It is well known that hydriding, followed by grinding, followed by dehydriding is commonly used to produce powders of some normally ductile pure elements such as Ti and Zr. In the case of Zr, for example, substantial mechanical grinding of the Zr hydride must be performed before the dehydriding step. Also high temperatures are required to hydride Zr. Conversely the gettering alloys used in this invention require neither elevated temperatures to hydride nor significant grinding to produce powder. These alloys react directly with H.sub.2 at room temperature and spontaneously form powder. Although hydride/dehydride techniques have been used to make Sm-Co and Nd-Fe-B magnet alloy powders, these techniques have not been used to manufacture Zr getter powders from alloys that are too tough to grind by conventional mechanical means.